Space-Time Transmit Diversity (MD) coding for two transmitting antennas has been adopted in many new wireless communication standards including 3GPP (3rd Generation Partnership Project), 3GPP2, and IEEE (Institute of Electrical and Electronics Engineers) 802.16, for example. It has been shown that so-called Alamouti codes for two antennas achieve maximum diversity gain for two transmit antennas and unity coding rate.
Numerous attempts have subsequently been made to search for space-time codes that achieve the maximum diversity gain with unity code rate for more than two antennas.
On the other hand, several studies on the combining of STTD and OTD (Orthonormal Transmit Diversity) have also been carried out. One primary difficulty associated with such coding schemes is that orthonormal complex matrices, which provide for optimal signal reception, have not been found for arbitrary numbers of transmit antennas. Although these combined codes possess simple encoding and decoding algorithms, in the absence of arbitrary-size orthonormal matrices, ad-hoc design of such codes is required, and results in sub-optimal performance.
One desirable aspect of STTD techniques is their applicability to single antenna receivers. Key advantages of STTD include maximum diversity gain and relatively simple decoding at a receiver involving only complex multiplications. In addition, while STTD is a complementary coding technique to MIMO (Multiple Input Multiple Output) BLAST, STTD does not require that the number of transmit antennas be less than the number of receive antennas across a communication network.
Therefore, although space-time techniques may provide advantages in communication systems, there are no currently known techniques that exhibit unity code rate and maximum transmit diversity gain for more than two transmit antennas.